This invention relates generally to a method and circuitry for generating drive pulses for a DC control element of the type having four semiconductor switches arranged in a bridge circuit, and more particularly, to a method and circuit wherein trigger pulses for the semiconductor switches are produced by comparing a triangular waveform with a control voltage; there being a pause between the "on" states of the semiconductor switches.
In known DC control elements, a resultant load current or load voltage is determined by the duty cycle of two semiconductor switches which are arranged diagonally opposite from one another in a bridge circuit. Load current and voltage is produced at the load in response to drive pulses which determine the conductive state of the semiconductor switches; the pulses having selectable duration. In accordance with the reference text, Netzgefuhrte Stromrichter mit Thyristoren (Line-Controlled Converters With Thyristors), by Gottfried Moltgen, 2nd. Ed., pages 278-9, such pulses are achieved by the comparison of a triangular, sawtooth voltage with a control voltage. The switching-on and switching-off states of the semiconductor switches are determined in response to the coincidence of the magnitude of the triangular sawtooth voltage waveform with a control voltage. In the known bridge circuit, it is important that the semiconductor switches which are connected in series in the bridge circuit not be simultaneously in conductive states, because such would be a short circuiting of the supply voltage source.
In the following discussion, the series circuit having two semiconductor switches connected across the power supply will be termed a "bridge branch." Two semiconductor switches connected to the same pole of the supply voltage are designated a "bridge half."
Short circuiting of the power supply is prevented by providing a pause between the "on" states of the semiconductor switches of the bridge. In known, commercially available equipment, each bridge branch is provided with first and second semiconductor switches which are operated inversely with respect to one another, and RC timing members are provided for producing the pause between the "on" states. Such an arrangement requires a large number of circuit components, and the duration of the pause is dependent upon the tolerance of the individual components in the delay circuit.
As a result of the pauses, which are required and are provided when switching from one bridge diagonal to the other, a dead zone is produced in the transfer characteristic such that small positive or negative control voltages cause no output voltage. The tolerance of the pause time, which is caused by the capacity of the capacitor in the RC circuit, produces the disadvantage that the dead zone is different for negative and positive values of control voltage. Thus, the transfer characteristic which correlates the control voltage and the output voltage of the DC control element is asymmetrical. As a result of this, the dead zone cannot be compensated uniformly for both polarities of control voltage at the zero crossing of the control voltage.
It is, therefore, an object of this invention to provide a method of generating drive pulses for a DC control element wherein the tolerance of the pauses can be maintained small. It is a further object of this invention to provide a method of generating drive pulses for a DC control element wherein pause times can be adjusted in a simple manner.
It is also an object of this invention to produce a simple circuit arrangement for generating drive pulses for a DC control element.